The Stationary Point: A New Method for Solar Wind Speed Measurements from a Moving Vantage Point

The WISPR imager on Parker Solar Probe (PSP) provides a unique view of the young solar wind as it flies through solar wind structures at high speed. It is of interest to use WISPR image sequences to measure the velocity of both large features (such as coronal mass ejections) and the background, ambient wind. However, WISPR's close-up, rapidly moving perspective makes the usual methods for measuring velocities from images difficult or impossible to apply, as most apparent motion through the image is due to the motion or rotation of the imager. In this work, we propose a new method of looking for features at the "stationary point"-a direction from which some plasma parcels appear to approach the spacecraft, remaining at a constant direction in the image sequence. This direction is a function of the plasma's radial velocity, the encounter geometry, and the spacecraft velocity, allowing the former two to be inferred. We demonstrate the technique with forward-modeled images, and we apply it to WISPR observations, inferring the speed and trajectory of a particular density feature. This method promises to enable speed measurements of the young solar wind in an important acceleration region, from a close-up perspective and at latitudes well outside the PSP orbital plane. And while we present this method in a solar wind context, it is broadly applicable to any situation of a moving viewpoint traveling through an expanding cloud of features.